Reflex extension of the crayfish abdomen is evoked by flexion of the mero-carpal joint of a single thoracic leg. Stimulation of the receptors which mediate this reflex should provide a method for the selective activation of the pre-motor interneurons which excite the abdominal motor centers. Single interneurons will be isolated in the nerve cord connectives and examined to see either if they are excited by stimulation of the leg receptors or if they excite the extension motor centers when stimulated electrically. Since these experiments will identify the pre-motor interneurons which are excited by leg stimulation, it will serve to test the command fiber theory of motor control.